Mike,
Really didn't notice the stamp until I took it out into the sunshine for pics a while ago. After looking at it through a glass, I feel that it is a stamp of some kind and not pitting or flaking that looks like a stamp. When and where it was stamped, I have no idea.
Could be a C or a G. I was wandering about the size of it too. (I guess size does matter)
I'm not going to try to pull the adapter out. Some guy whacked it in there 150 years or so ago, and I don't feel right pulling it out just to satisfy my curiosity. It needs to stay where it is.
Thanks to all.
Dan